The present invention relates generally to columns in which mass transfer and heat exchange occur and, more particularly, to liquid distributors used in such columns and methods of liquid distribution using such liquid distributors.
As used herein, the term “mass transfer column” refers to a column in which mass transfer and/or heat exchanger occur. Examples of mass transfer columns include distillation, absorption, stripping, and extraction columns.
In mass transfer columns, one or more liquid and/or vapor streams are brought into contact with each other to effect mass transfer and/or heat exchange between the liquid and/or vapor streams. Beds of structured or random packing are normally used in such mass transfer columns to facilitate intimate contact between the liquid and/or vapor streams and thereby enhance the desired mass transfer and/or heat exchange between the streams. In liquid/vapor systems, the liquid stream descends through the bed of packing and the vapor stream ascends through the packing bed. Similarly, in liquid/liquid and vapor/vapor systems, the denser phase descends through the bed and the less dense phase ascends through the bed.
Uniform distribution of the descending liquid stream across the horizontal cross section of the bed of structured or random packing is important in order to maintain a uniform interaction between the liquid stream and the ascending vapor stream. Various types of liquid distributors are used in an attempt to provide a uniform distribution of the liquid stream as it enters the top of the bed of packing material. In one type of liquid distributor, a feed box or parting box receives a liquid stream from an overlying collector or a feed line and distributes it to a number of elongated and parallel troughs that underlie or extend horizontally from the parting box. Spaced-apart holes are formed in the side walls of the troughs to allow liquid to exit the troughs in individual liquid streams. Splash baffles are spaced outwardly from and parallel to the side walls of the troughs so that the individual liquid streams exiting the troughs through the holes are directed onto the splash baffles. The individual liquid streams then descend along and spread across the splash baffles before dripping off the lower edge of the baffles into the bed of packing material. Examples of liquid distributors of this type are shown in U.S. Pat. Nos. 6,722,639 and 7,125,004.
When designing the liquid distributors described above, the number and size of the holes in the side walls of the trough is selected based on the anticipated volumetric flow rate of the liquid stream into the troughs. The total open area presented by the holes must be designed to permit a sufficient liquid head to develop within the troughs and thereby generate the necessary force to cause the individual liquid streams to exit the holes with enough momentum to reach the outwardly-spaced splash baffles. When the designed liquid volumetric flow rate is low, the total flow capacity of the holes must be reduced to allow a sufficient liquid head to develop in the troughs. This reduction in flow capacity can be achieved by reducing the size of the holes and/or by increasing the spacing between adjacent holes to reduce the total number of holes. Both of these options create potential disadvantages. If smaller holes are selected, they are more likely to become clogged, thereby creating regions on the splash baffles and in the underling packing that are not wetted by the individual liquid streams. Similarly, if the spacing between the holes is increased, the individual liquid streams may not merge as they descend along and spread across the splash baffles. A need has thus developed for a liquid distributor that overcomes these potential disadvantages.